Imagine you were a Hollywood producer pitched the following idea: a baby girl born into an acting dynasty is put to work in a dog food commercial at the age of 11 months. At seven, she’s a film star pouring Baileys over her ice-cream, at 11 she develops a drink problem, at 12 she’s a drug addict, at 13 she cuts her wrists and is hospitalised, and at 14 she’s legally divorced from her parents. Of course, you wouldn’t make the movie. Too far-fetched. There’s only so much disbelief that one can willingly suspend.

But you’ve not heard the half of Drew Barrymore’s story. Unemployable as an actor at 15, cleaning toilets at 16, she was twice married and twice divorced by her mid-20s. Now she has written a book called Wildflower, a not-quite memoir; low-key essays that travel back and forth in time, telling stories of her outlandish life. There’s a lot of hippy-dippy philosophising and sugary existentialism, but it’s also very moving. More than anything, it’s a book about the lost, loveless girl who finally finds a family and love.

Barrymore is now a youthful 40 – diddy (5ft 3in), pretty, exuding good health. She is wearing jeans, a stripy top and flat sandals, and has a bottle of beer in her hand. “Cheers,” she says. We clink bottles. She has just raced through a photoshoot in downtown Manhattan with turbo-charged efficiency. OK, you want happy, sad, stupid, funny? Her face changes by the second. In the book, she talks about the ways she has changed since having children. Before, she was no respecter of time, invariably late for appointments. Today, she’s still half an hour late, but apologetic and keen to make amends.

Reading Wildflower, I thought of Never Been Kissed, the first movie Barrymore made with her production company, Flower Films, and one of her most successful. In it, she plays Josie Geller, an aspiring magazine writer given an undercover reporting job: to pretend to be a high school student to find out what modern schools are like. Josie returns to school and is quickly reminded of its horrors – as a girl she had been a cack-handed clever clogs, ostracised by the cool crowd, taunted by the boys, and known as Josie Grossie. When she returns undercover, little has changed. But, as is the way with romcoms, the underdog wins the day.

Never Been Kissed became the modern-day fairytale for a generation of teenage girls. An admission: it has been the soundtrack to my family life for the past 15 years. Virtually every time my elder daughter, Alix, invited friends round, they would watch the film. Now aged 23, and a teacher, she says she has watched it more than 50 times.

Barrymore smiles when I tell her this. “Wow! I love that. I am Josie Grossie. If you say that to your daughter, she’ll understand.” She talks about her feeling of not belonging, her clumsiness, her fixation with words, her overeagerness to correct others.

But the younger Barrymore was ridiculously cute. Although her peers might not have liked her (she says she could never relate to other children), movie-goers, young and old, adored her. She made her debut at five, in the Ken Russell sci-fi horror Altered States, but it was Steven Spielberg’s ET, two years later – the fourth most successful film of all time – that made her famous. In a film of cuties (the little boy Elliott, ET himself), Barrymore’s pigtailed, open-mouthed Gertie out-cuted them all, her initial terror evolving into something approaching sibling love.

In the years immediately after ET, she started getting into trouble. There’s a famous clip of her being interviewed by Johnny Carson around this time. She is seven going on 27, wearing fake front teeth to cover up the milk teeth she has just lost, which she quickly discards and dumps on his desk. She is precocious, funny, and outrageously flirtatious with the middle-aged chatshow host.

What we didn’t know at the time was that her father, the actor John Drew Barrymore, was a violent alcoholic, and her mother and manager, Jaid, who was born in a displaced people’s camp in Germany to Hungarian second world war refugees, was herself a wild child with little concept of parental responsibility. After her parents divorced, when Drew was nine, Jaid took her to Studio 54, where she was introduced to drugs and encouraged to dance with famous young men.

From the age of eight, she called herself a “party girl”, going out with her mother and her mother’s friends up to five times a week. But she soon couldn’t cope. By the age of 12, she had already been in rehab and was supporting Nancy Reagan’s Just Say No campaign. She fell off the wagon again, and at 13 found herself beginning an 18-month stint in hospital, where she was treated for alcohol and drug addiction.

I ask Barrymore whether her 14-year-old self would have imagined being able to tell such a positive life story at 40. She takes a swig from her bottle of Corona. “Half no, in that I was so scared of not knowing where I was going. I really had a fear that I was going to die at 25. And half yes, because no matter how dark shit got, I always had a sense that there should be goodness. I never went all the way into darkness. There were so many things I could have done that would have pushed me over the edge and I just knew not to go there.”

But Barrymore couldn’t have come much closer. What was her nadir? “When I was 13, that was probably the lowest.” What happened then? She looks away. “Just knowing that I really was alone. And it felt… terrible. It was a really rebellious time. I would run off. I was very, very angry.”

What was she most angry about? Silence. “That’s the thing. I don’t know. And once I really asked myself, ‘What are you angry with?’ I dropped the anger. If you search deep down in me, it’s like, why am I so angry, man? And it’s like, OK, cos my parents weren’t there, who gives a shit? Lots of people don’t have parents. They were gone, they couldn’t handle any of it, and I get it.”

Yes, lots of parents are not there for their children, I say, but few are quite as “gone” as yours were. She grins. “They were pretty out there! But I realised, honestly, yeah, my mom locked me up in an institution. Boo hoo! But it did give an amazing discipline. It was like serious recruitment training and boot camp, and it was horrible and dark and very long-lived, a year and a half, but I needed it. I needed that whole insane discipline. My life was not normal. I was not a kid in school with normal circumstances. There was something very abnormal, and I needed some severe shift.”

I have heard Barrymore refer to this institution before, but have never been sure what it was exactly. Was it an institution for the mentally ill?

“Yeah, absolutely.”

Could you leave or did you have to stay?

“Oh yeah. No getting out for a year and a half.”

Was there any warning from your mother that this was coming?

“No, no, no. I would have run away. I would never, never have let that happen to myself.”

Did she come and visit?

“Yeah, occasionally. Occasionally.”

It’s a disturbing story. She talks about life in the dormitories, how she is still friends with one girl who “is kooky, but great”, how they tried to drug her and she resisted. “I was like, no, thank you. I wanted to clean out. I didn’t want to be a cliche.”

Did you ask your mother why she sent you there?

“Of course, but I got it. Then we emancipated. We separated after that. I legally became an adult.”

Did you ever think you were mentally ill? “No. No. I just knew that I was off course.”

Did the doctors say what they thought was wrong? She giggles. “Oh, they definitely thought I was off course!”

In the end, it was the institution that suggested she legally separate from her mother and be declared an adult at 14. Barrymore says the experts there believed that if she were to go back out into the world, she’d be better off on her own. Today, she has no regrets about her time there. “It was a very important thing to experience for me. It was very humbling, very quieting. Maybe it was necessary, because I came out of there a more respecting person. And my parents didn’t teach me that, and life wasn’t teaching me that. I came out in a very different way… but I still was me.”

I ask Barrymore if she enjoyed any of her child stardom. She says she’s not sure. “I don’t think I understood what was good, or pleasurable, or bad. I was probably chasing joy, but I don’t think it was the real joy. I was just too young to know.”

Did you feel exploited by your parents? “Nooooo.” She starts again. “I mean, well, yeah, I think with my mother it was definitely too out there. But my dad, no, he was just unavailable.”

The newly independent 14-year-old was a Hollywood pariah. A has-been. She’d attend auditions and casting directors would laugh at her nerve for even turning up. “To have such a big career at such a young age, then nothing for years – people going, you’re an unemployable disaster – that’s a tough trip to have by the time you’re 14. To have access to so many things, then to nothing.”

She stops, and says maybe that should have felt terribly unfair, but it didn’t. She just accepted it, had no ego about it – she couldn’t afford to have. One of the few useful lessons her father taught her, she says, was about how ruinous expectation can be. “My dad once said to me that expectations are the mother of deformity, and I do not expect anything. Expectations always got me in trouble. What do expectations really do? They make other people feel like shit, then let you down eventually.” Far better, she says, to get on with things and fight back. So she worked in restaurants and cleaned toilets and told people that, yes, she used to be Drew Barrymore and still was.

As she talks, I look at two tattoos, one on each arm. On her left, the word BREATHE, vertically, in stencil-like capitals (“You’re never worse after a deep breath,” she says); on the right is a tiny bird. When she was a little girl, she asked her mother if Steven Spielberg, who directed her in ET, could be her godfather. Spielberg agreed. Did she call on him for advice in her toughest times? “I didn’t want to show him certain things. He has always inspired me to be my best, so I didn’t want him to see me at my worst. I could have easily gone to him, he was never closed doors or unwelcoming. But I was like, I’ll figure this out over here, I will be back, excuse me for a minute!” (After she posed naked for Playboy, aged 19, Spielberg sent her a large quilt with a note attached that read, “Cover up”.)

She finishes her beer, and for the first time looks slightly uneasy. “By the way, we are talking about all these things that aren’t in the book.” She says I am focusing on the unremittingly bleak, and the book talks about precious, private moments of hope. Such as? “Being on a boat and asking the universe not to give up on me. Or how I felt walking away from my mom at 14, and what that first year was like. It was weeeeeird. I had no idea how to run an apartment at 14. There was fungus growing everywhere, it was a disaster. It was in a dangerous neighbourhood and I was so scared to sleep. I had bars on the window and alley cats fucking 30 feet away. I was so terrified.”

To be honest, her positive moments don’t sound much less bleak than anything else we’ve talked about. But, she says, in the book she deliberately didn’t go into explicit detail about past problems. “This was aimed at my kids reading it one day, so there’s a demure quality to it.” Her daughters, Olive and Frankie, with husband Will Kopelman, are three and one. Does she want to protect them from her past? She flinches. “No, it’s not denying anything. I’ve been caught off guard when people go, ‘What are you going to do when your kids Google you?’ and I’m like, ‘God, that is so accusatory.’ I’m not going to pretend I am not who I am. I’m going to show them how it got me to where I am now.”

We get a couple more beers. I ask how long she was a Hollywood untouchable for. Ages, she says, maybe eight years. I’m sure it wasn’t that long. So she counts on her fingers, and is surprised to discover she was only persona non grata for around three years. By the age of 17, she was back with Poison Ivy, playing a character close to her own public image – sexy, trashy, dangerous. Over the next six years she made 16 more films, including Bad Girls, Woody Allen’s musical Everyone Says I Love You, the blockbuster Batman Forever, the horror movie Scream and The Wedding Singer, the first in a series of romcom collaborations with Adam Sandler. At 20, she joined forces with Nancy Juvonen to start Flower Films, producing and starring in Never Been Kissed three years later.

She was tired of playing bad girls; had never really seen herself as one. It gave her the scope to choose her own parts – whether the misunderstood Cinderellas of the romcoms or the kick-ass heroine of Charlie’s Angels.

She found her 20s liberating – working hard, partying hard, enjoying huge success and a belated adolescence. At one point, she went on David Letterman, jumped on the chatshow host’s desk, gave him a table-dance, flashed her breasts and jumped back into her seat grinning like crazy, wondering what the hell she had just done. You couldn’t tell whether it was with shame or pride, but it was noticeable that, at 20, she seemed younger than the seven-year-old who had appeared on the Johnny Carson show all those years ago.

She says her 20s and 30s more than made up for her teens. “Twenty to 35 was a blaaaaast. I thought, how am I getting away with this? I’m really being quite playful, yet still doing a lot at work.”

During this time, she briefly married for a second time, to comedian Tom Green (she had previously been married, at 19, to Welsh bar owner Jeremy Thomas), and had a number of relationships, including a long-term one with the Strokes drummer Fabrizio Moretti. Is “playful” a euphemism for sex, drugs and rock’n’roll? “No,” she says. “More like travel, really fun times with friends, relationships, sure. But I really lived, and did what I wanted to when I wanted to. If I felt like doing something, I just did it. And it was pretty liberating. I wasn’t like a nun who went to sleep at 10pm every night. I had fun!”

It’s interesting, I say, that despite your earlier excesses, you went on another blast. A different character might have become abstemious. She could never do that, she says. “I can’t have more severity in my life. I think I’ve had a lot of severity, and it’s not for me. I like moderation or balance.”

In 2009, she directed her first full-length feature film, Whip It, a typical Barrymore romcom about a teenage misfit who finds herself by signing up to a roller derby team. The film received generally positive reviews (critic Roger Ebert wrote, “While it may not reflect the kind of female empowerment Gloria Steinem had in mind, it has guts, charm, and a black-and-blue sweetness”), but it wasn’t a box-office success.

What has given her the most satisfaction professionally: acting, producing or directing? “Having a hand in it, being part of the process, has given me tremendous satisfaction. I don’t like just showing up… I’ve never been good at, ‘Hope it all goes well.’ I want to be a part of why it goes well. I’m not blind when I walk into things. I’ve done my homework. I like to be part of something. I care. I care about the details.”

She is extremely proud of her acting lineage – seven generations of actors, including her celebrated grandfather John Barrymore, who eventually drank himself to death (alcoholism is another family trait). But while she loves acting, she is dismissive of her own abilities. In Wildflower, Barrymore suggests she has pretty much always played herself – so when she was lovelorn and misunderstood she was perfect for the romcoms, but now she’s a contented mother she’s fit only for playing contented mothers, and they are pretty dull roles. She refers to most of the characters she has played in recent years as “panting labradors”. She pants enthusiastically to show what she means. “Like a labrador, panting on the floor. It’s eagerness.” Eager to please, eager to be loved.

Barrymore insists she has neither the time nor inclination to play demanding roles these days. This year she starred in the comedy drama Miss You Already, with Toni Collette, about best buddies who have shared everything, including boyfriends; last year she played opposite Adam Sandler in the twee romcom Blended. She admits she has only ever truly stretched herself once as an actor, in the 2009 HBO film Grey Gardens, playing Edith Bouvier Beale, the reclusive cousin of Jackie Kennedy.

“Grey Gardens was a big buy for me. It was like, right, we’re done for a while, because I went so cuckoo on that. I got to do the dream. I got to have 17 layers of chicken skin on my face. It took four hours a day for them to make me look like this woman. I didn’t talk to anyone for four months, and only spoke like her. I wouldn’t be able to do that now. What am I going to tell my kids? ‘Sorry, I can’t speak to you for four months, because I’ve got to be Edie Beale.’” She says it was hugely important for her, because she wanted to prove to herself and the film industry that she could do serious. “The director didn’t even want me for that movie. He was like, ‘Oh no, please not her, not the romcom girl.’ And I was like, ‘I can do it! I caaaaan do it.’” She has such a distinctive voice – those creamy California vowels stretched to breaking point, as if talking while chewing on treacle.

Barrymore says that since having children, her priorities have changed. “Without pissing on what I’ve done, I think I really did have a desperation – I felt that everything I did in film mattered. It was my whole world. Now it’s kids, friends, marriage, work, health. I don’t want my girls to grow up saying, ‘Oh wow, yeah, she really worked hard, but I didn’t see her.’ I want them to be like, ‘I don’t know how the hell she was there for all those things, and she still worked!’”

Did she worry about what she would be like as a mother? “No. I knew I would not repeat the mistakes of my parents. I knew I would never do that to a kid. I wouldn’t not be there, or put them in too-adult circumstances. I knew I’d be very traditional, or I would not do it. I would never have had children unless I was incredibly stable, and willing to put them first.” As she explains the wonder of motherhood, the words tumble out so quickly, she trips over them. “It really is the most clever, smart, capable, patient, loving, creative, agile thing you will ever do when you’re alive. It’s amazing. So I just wanted to be present for that. I also waited. I knew I wouldn’t do it till I was ready.”

How would she feel if her girls wanted to become child film stars? “I would unfortunately have to risk them hating me.” You wouldn’t let them do it? “No, I wouldn’t. That doesn’t mean I would ever shit on the profession of acting. I think it’s wonderful. I think films saved my life. I mean, I come from a family that has done acting for 400 years. But film sets are a bizarre world. For me, it was better than my circumstances. It was a saviour. For my children, it will not be better than their circumstances. They are going to be so safe and so loved that they won’t need a film set to make their life better.”

After Barrymore announced that she would rather stay at home with her children than work on movie sets (albeit still focusing on her successful beauty product business), she experienced a backlash. From whom? “Women! For saying, you can’t have it all. But I didn’t mean it like that. I think you most certainly can do anything you want, I just think… I can’t do everything at once. It isn’t going to warrant a good result and isn’t actually possible. And that really pissed people off.” Was she upset by the response? “No, but I felt it was misunderstood. I think women know how much I am for women and about women, but I do have an issue with, ‘You can have it all.’ It’s an impossible expectation to put upon yourself. And what does having it all actually mean? It sounds very greedy, you know, ‘I can have it all.’ I can’t have it all.”